In the model they consider, the star is initially part of a binary system at the center of our galaxy. The binary system passes close enough to the supermassive black hole, Sag A*, so that the subgiant is captured in close orbit while its companion escapes. Over time, the orbit of the subgiant decays and the star starts to enter the danger zone of Sag A*. This is where things get interesting.

Because the outer layers of the subgiant are somewhat swollen, they are the first to be captured by the black hole. Essentially, the black hole can rip off the outer layers of the star, leaving a dense helium core. This bare core star continues to orbit ever closer to the black hole until finally being consumed.